As a result of the Internet's tremendous growth as well as the proliferation of various types of mobile communication devices, data transfers are increasing dramatically. In addition, data streams for digital video, high-definition television (HDTV), and color graphics are requiring higher and higher bandwidth. The digital communications deluge is the driving force for high-speed interconnects between chips, functional boards, and systems.
In computer and information processing systems, various integrated circuit chips must communicate digitally with each other over common buses. The receiving bus nodes recognize the signal as being high or low using receivers, which are also referred to as input buffers. Often the receiver is a differential receiver; that is a receiver that detects the difference between two input signals, referred to as the differential inputs. These input signals may be a received signal and a reference voltage or they may be a received signal and the inverse of the received signal. In either case, it is the difference between the two input signals that the receiver detects in order to determine the state of the received signal.
Interface circuits facilitate the serial transmission of digital information between a transmission source, such as a computer or a handheld mobile device, and a transmission destination, such as a modem, a printer, another computer, or another handheld mobile device. Interface circuits include transmission line drivers, which convert data signals of a first form (e.g., LVDS) into RS (recommended standard) signals that are transmitted on data transmission lines, and receivers, which convert the RS signals into data signals acceptable to a designated device (e.g., TTL). For high speed serial communications over electrical interconnections, controlled impedance transmission lines are used to maximize the available data rate. In order to minimize the impact of reflections on the received signal integrity, it is desirable to terminate the transmission lines at the receiver inputs with a termination that is matched to the characteristic impedance of the transmission line. For differential signaling, two modes of operation exist for the received signals, the differential and common mode. Differential signaling refers to the difference between two signals carrying equivalent but complementary data on a pair of matched impedance transmission lines. Common mode signals refer to a signal that is received simultaneously on both of the differential inputs. These common mode signals can be caused by noise or crosstalk injected onto both of the differential transmission lines. For maximum signal integrity, it is desirable for both the differential and common mode signals to be properly terminated at the differential inputs.